Brian O’Kelley, the founder and chief executive officer of AppNexus, the ad-tech company recently bought by
AT&T Inc.
T -0.37%
for around $1.6 billion, is shifting to an advisory role at the telecoms firm, according to people familiar with the matter.

AT&T’s acquisition of AppNexus helped lay the foundations for a new advertising and analytics unit at AT&T called Xandr. That unit is led by Brian Lesser, a former executive at
WPP
WPP 0.38%
PLC’s GroupM media-buying unit, who is planning to build a first-of-its-kind marketplace for television and digital video advertising.

In an email to employees reviewed by The Wall Street Journal, Mr. Lesser said Mr. O’Kelley will move away from day-to-day operations and work in an advisory role “for the foreseeable future.”

“Brian is a leader’s leader, and I appreciate him setting up a fantastic team at AppNexus that will help lead Xandr to the next level,” Mr. Lesser said in the email.

Xandr—its name is derived from that of AT&T founder Alexander Graham Bell—also includes AT&T’s AdWorks and data-and-analytics units.

When AT&T’s purchase of AppNexus was first reported, Madison Avenue executives predicted the platform could offer them more choice in the digital ad market and provide more competition with the industry’s dominant players, Google and
Facebook Inc.

Jonathan Hsu, the former chief financial officer of AppNexus, and AppNexus’s former chief people and legal officer, Nithya Das, are also moving to advisory roles.

Mr. O’Kelley founded AppNexus in 2007 and built it into one of the largest independent ad-tech companies. Before that he was the chief technology at Right Media, an online advertising company that sold to
Yahoo
in 2007 for $680 million.

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